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Date:   Wed, 29 Sep 2021 19:36:11 +0200
From:   Alexey Gladkov <legion@kernel.org>
To:     Jordan Glover <Golden_Miller83@protonmail.ch>
Cc:     ebiederm@xmission.com, LKML <linux-kernel@vger.kernel.org>,
        "linux-mm\\@kvack.org" <linux-mm@kvack.org>,
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        <containers@lists.linux-foundation.org>,
        Yu Zhao <yuzhao@google.com>
Subject: Re: linux 5.14.3: free_user_ns causes NULL pointer dereference
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On Tue, Sep 28, 2021 at 01:40:48PM +0000, Jordan Glover wrote:
> On Thursday, September 16th, 2021 at 5:30 PM, <ebiederm@xmission.com> wrote:
> 
> > Jordan Glover Golden_Miller83@protonmail.ch writes:
> >
> > > On Wednesday, September 15th, 2021 at 10:42 PM, Jordan Glover Golden_Miller83@protonmail.ch wrote:
> > >
> > > > I had about 2 containerized (flatpak/bubblewrap) apps (browser + music player) running . I quickly closed them with intent to shutdown the system but instead get the freeze and had to use magic sysrq to reboot. System logs end with what I posted and before there is nothing suspicious.
> > > >
> > > > Maybe it's some random fluke. I'll reply if I hit it again.
> > >
> > > Heh, it jut happened again. This time closing firefox alone had such
> > > effect:
> >
> > Ok. It looks like he have a couple of folks seeing issues here.
> > I thought we had all of the issues sorted out for the release of v5.14,
> > but it looks like there is still some little bug left.
> >
> > If Alex doesn't beat me to it I will see if I can come up with a
> > debugging patch to make it easy to help track down where the reference
> > count is going wrong. It will be a little bit as my brain is mush at
> > the moment.
> >
> > Eric
> 
> As the issue persist in 5.14.7 I would be very interested in such patch.
> 
> For now the thing is mostly reproducible when I close several tabs in ff then
> close the browser in short period of time. When I close tabs then wait out
> a bit then close the browser it doesn't happen so I guess some interrupted
> cleanup triggers it.

I'm still investigating, but I would like to rule out one option.
Could you check out the patch?

diff --git a/kernel/ucount.c b/kernel/ucount.c
index bb51849e6375..f23f906f4f62 100644
--- a/kernel/ucount.c
+++ b/kernel/ucount.c
@@ -201,11 +201,14 @@ void put_ucounts(struct ucounts *ucounts)
 {
        unsigned long flags;

-       if (atomic_dec_and_lock_irqsave(&ucounts->count, &ucounts_lock, flags)) {
+       spin_lock_irqsave(&ucounts_lock, flags);
+       if (atomic_dec_and_test(&ucounts->count)) {
                hlist_del_init(&ucounts->node);
                spin_unlock_irqrestore(&ucounts_lock, flags);
                kfree(ucounts);
+               return;
        }
+       spin_unlock_irqrestore(&ucounts_lock, flags);
 }

 static inline bool atomic_long_inc_below(atomic_long_t *v, int u)

-- 
Rgrds, legion